1. Field of the Invention
The present invention relates to a solid state image sensor, and more particularly, to a solid state image sensor and a method for fabricating the same, which can improve a device sensitivity and allows an effective individualization of chips.
2. Background of the Related Art
Being a photoelectric conversion device which converts an optical signal into an electric signal, the solid state image sensor is in general provided with a microlens over each of photoelectric conversion area for focusing a light. The focused light is directed onto the photoelectric conversion area, and converted into a video charge corresponding to the directed light. For an effective focusing of the light, a curvature, a refractive index, and a focal distance of the microlens should be precisely controlled. That is, in order to improve a picture taking sensitivity, what should be observed are, first, a precise focusing control of the light onto the photoelectric conversion area, second, an optimization of a microlens curvature, and third, elimination of contamination during formation.
A related art solid state image sensor will be explained with reference to the attached drawings. FIG. 1 illustrates a section of a related art solid state image sensor, and FIG. 2 illustrates a system of focal distance calculation of a microlens.
The related art solid state image sensor is provided with a first p type well 2 formed in a surface of an n type semiconductor substrate 1, and a photodiode region 4 formed in the first p type well 2 region. And, there are vertical charge coupled device regions 6 between the photodiode regions 4 in a vertical direction for transferring signal charges generated in the photodiode regions 4 in the vertical direction. There is a second p type well 3 having p type ions implanted therein beneath the vertical charge coupled region 6 for improving a charge transfer efficiency, and there is a channel stop layer 5 between the photodiode regions 4 and the vertical charge coupled regions 6 for electrical separation of the regions. There is a surface P.sup.+ layer on a surface of each of the photodiode regions 4 for improving a charge generation efficiency. And, there are polygate electrodes 8 formed repeatedly on the vertical charge coupled device regions 6 insulated by an interlayer insulating layer 7, and there a metal light shielding layer 9 for preventing smear on regions excluding the photodiode regions 4 at which actual light focusing is made. There is a planarizing layer 10 on an entire surface of the metal light shielding layer 9, and a color filter layer 11 on the planarizing layer 10 opposite to each of the photodiode regions 4 for only transmission of a specific wavelength. There is a top coat 12 on an entire surface inclusive of the color filter layer 11, and there is a microlens 13 formed on the top coat 12 opposite to each of the photodiode regions 4.
A focal distance of the aforementioned related art solid state image sensor is determined by the following method.
Referring to FIG. 2, it can be known that the focal distance of the microlens is fixed by a height `t'` of the microlens, a distance `t` between a surface of the photodiode and a bottom plane of the microlens, and refractive indices of air and the microlens n.sub.0, and n.sub.1. For example, if a light is incident to the surface of the photodiode region vertically, the focal distance `f` is defined as follows. ##EQU1##
If `t` denotes a distance from the surface of the photodiode to the bottom plane of the microlens, as r.sup.2 =p+.RTM.-t').sup.2 and t=f-t', the `t` can be expressed as shown below. ##EQU2##
Where p is a horizontal distance between cells divided by 2.
Thus, the `t` has a substantial margin, wherein, if a horizontal margin is `d`, a vertical margin Vt can be expressed as dt/2p. Enhancing focusing of light onto the photodiode region by precise focal distance control using the above factors is an important factor in improving the picture taking sensitivity. Other than the focal distance control, optimization of a microlens curvature and a contamination reduction are also important in improving the picture taking sensitivity.
Upon completion of fabrication of the related art solid state image sensor thus, a chip individualization process is proceeded as follows. FIGS. 3A.about.3F illustrate sections showing the steps of fabrication of the related art solid state image sensor. After completion of device fabrication process done on a wafer, the following packaging process is proceeded.
Referring to FIG. 3A, after a UV tape 32 is attached to an entire surface of the wafer 31 having individual chips fabricated thereon, the wafer is subjected to die sawing along a scribe line in the wafer 31 using a sawing machine 33. Foreign matters 34 in a nature of a resin fallen off from the UV tape 32 during the die sawing are present on a surface of the wafer 31. Then, as shown in FIG. 3B, a UV ray is directed to the chip of which die sawing is completed for easy removal of the chip from the UV tape 32. As shown in FIG. 3C which illustrates a partially enlarged section of an individual chip portion, the direction of UV ray hardens an adhesive composition 36 in the UV tape 32 in contact with the microlens, which weakens an adhesive force of the adhesive composition 36. An initial adhesive force of the adhesive composition on the UV tape 32 is in a range of 300.about.400 g/25 mm. It is found that the direction of UV ray to the UV tape in a non-actual fabrication state reduces the adhesive force down to 1.about.30 g/25 mm. However, the reduction of the adhesive force in an actual fabrication is not so great, but is in a range of 50.about.100 g/25 mm. Because the microlens is formed of a resin of a polyimide group, and the adhesive composition on the UV tape is a photoresist resin of the same group. As shown in FIG. 4D, after a remove tape 37 is attached to the UV tape 32 used for supporting the chip in the die sawing, the UV tape 32 attached to the surface of the wafer 31 is removed using the remove tape 37. As shown in FIG. 3E, because the UV tape 32 is removed in a state a complete hardening of the adhesive composition is not fulfilled, there are foreign matters 34 in a nature of resin from the UV adhesive agent and particles 38 of the UV tape are present on the surface of the wafer 31. As shown in FIG. 3F which is a partially enlarged view centered on the microlens 35, those foreign matters are still present even in an individual chip state after finish of the die sawing.
The aforementioned related art solid state image sensor has the following problems.
First, there is a limitation in focusing a light because the focusing is fully dependent on a refraction of the microlens in the process that the light is focused through the microlens and directed to the photodiode. This limitation comes from a limitation of a refractive index of a material of the microlens itself and a limitation of a refractive index of the microlens coming from different fabrication conditions considering the curvature and the like.
Second, there is a gap of approx. 2 .mu.m provided between adjacent microlenses for obtaining a required curvature of the microlens. If the gap is not provided, but the microlenses are formed bringing the microlenses to full contact, there will be a deep groove formed between adjacent microlenses, which makes removal of the foreign matters difficult. The light incident to the gap of the microlenses is not focused by the microlenses. Accordingly, the presence of gap between the microlenses drops an effective light focusing efficiency.
Third, though the adhesive force of the UV tape should be reduced in the UV tape removal step for die attachment after the die sawing, the reduction of the adhesive force is difficult because the UV tape is formed of a polyimide group identical to a material of the microlens, to leave foreign matters in a nature of resin, silicon dusts, and UV tape debris on the microlens in the UV tape removal step using the remove tape, which causes formation of scratch on the microlens.
The related art solid state image sensor has a low sensitivity and yield caused by the above problems.